Tiffany is 18 years older than Omar. For the last two years, Tiffany and Omar have been going to the same school. Fifteen years ago, Tiffany was 4 times older than Omar. How old is Tiffany now?
Explanation: We can use the given information to write down two equations that describe the ages of Tiffany and Omar. Let Tiffany's current age be $t$ and Omar's current age be $o$ The information in the first sentence can be expressed in the following equation: $t = o + 18$ Fifteen years ago, Tiffany was $t - 15$ years old, and Omar was $o - 15$ years old. The information in the second sentence can be expressed in the following equation: $t - 15 = 4(o - 15)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $t$ , it might be easiest to solve our first equation for $o$ and substitute it into our second equation. Solving our first equation for $o$ , we get: $o = t - 18$ . Substituting this into our second equation, we get the equation: $t - 15 = 4($ $(t - 18)$ $ -$ $ 15)$ which combines the information about $t$ from both of our original equations. Simplifying the right side of this equation, we get: $t - 15 = 4t - 132$ Solving for $t$ , we get: $3 t = 117$ $t = 39$.